Wave transmission lines and networks – Coupling networks – Delay lines including long line elements
Reexamination Certificate
2001-03-07
2003-01-14
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Delay lines including long line elements
C330S151000, C333S161000, C333S117000
Reexamination Certificate
active
06507253
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
Present invention relates to a delay circuit and a feedforward amplifier, particularly relates to a delay circuit and a feedforward amplifier used for an amplifier in the base stations of mobile communication systems that include cellular phones or the like.
2. Description of Related Art
In recent years, for the transmission system for the mobile communication base stations, in order to amplify a plurality of signal channels as a whole, an effective power amplifier excellent in linearity has been required. Carrying out distortion compensation in a manner of feedforward provides the linearity. (Reference: John L. B. Walker, “High-Power GaAs FET Amplifiers” Artech House, 1993, pp332-333)
In the feedforward amplifier, in order to coincide the signals from different signal paths with each other, a delay circuit is required. When manufacturing a feedforward amplifier, as it is necessary to adjust the delay time of the delay circuit, it is required that the delay time of the delay circuit can be changed easily.
As a delay circuit that satisfies the requirements like these, a coaxial cable has been used. In the coaxial cable, as the length and the delay time are in a proportional relationship, by adjusting the length of the coaxial cable, fine adjustment of the delay time can be made easily.
Whereas, referring to
FIG. 10
, a description will be made about a conventional delay circuit that is used in a circuit other than the feedforward amplifiers described above.
In
FIG. 10
, reference numeral
201
denotes an input terminal;
202
denotes an output terminal;
203
denotes a circulator; and the terminals are denoted as terminal-a, terminal-b and terminal-c. Reference numeral
204
denotes an open-ended transmission line comprised of, for example, a microstrip line or the like.
In the circulator
203
, it is assumed that an input signals Sa inputted from the terminal-a is outputted to the terminal-b;
an input signals Sb inputted from the terminal-b is outputted to the terminal-c; and an input signals Sc inputted from the terminal-c is outputted to the terminal-a. In the structure shown in
FIG. 10
, as
201
is the input terminal and
202
is the output terminal, the input signal Sc does not exist actually.
In case where the terminals of the circulator
203
are sufficiently isolated (that is, in case where the signals are not transmitted from terminal-c to terminal-b; from terminal-b to terminal-a; and from terminal-a to terminal-c), the signals inputted from the input terminal
201
are transmitted along the transmission line
204
through terminal-a and terminal-b of the circulator
203
, reflected totally at the open-end of the transmission line, returned along the transmission line
204
again, and are outputted to the output terminal
202
through terminal-b and terminal-c. Delay time of the input signals can be changed by changing the length of the transmission line
204
. Accordingly, conventionally, in order to prepare a delay circuit having a desired delay time, the method described below has been used.
As an example thereof, a plurality of transmission lines of different length are previously prepared. And then, they are connected one by one and delay time is measured each time. Thus, the steps are repeated until a desired delay time is obtained.
Further, as another example, a plurality of delay circuits are previously prepared by connecting transmission lines of different length respectively. And then, by measuring delay time of the prepared delay circuits, the delay circuits are sorted into groups having a specific range of delay time respectively. In this manner, a plurality of delay circuits that have different specified delay times are prepared simultaneously. When a request for a delay circuit having specific delay time is made, a delay circuit that satisfies the requirements is provided from a plurality of groups of delay time.
In such a manner as described above, even when the relationship between the length of the transmission line
204
and delay time of the signals is not cleared, it is possible to manufacture desired delay circuits. Also, because the delay circuits prepared as described above are, different from the coaxial cables described above, used under a situation where a fine adjustment of delay time is not necessary, no one have directed attention to a point that, between the above-mentioned length of the transmission line and the delay time of the signals, what relationship exists. Furthermore, in actual circulators, terminals are not isolated sufficiently. For example, it is known that a part of components of signals out of the signals inputted from the terminal-a is outputted directly to the terminal-c. However, no one has known whether such leakage signals render adverse effects or not. However, even when no one knows about that, no problem was recognized.
Under such a circumstances as described above, the inventor of the invention thought whether it is possible or not to use the above-mentioned delay circuit shown in
FIG. 10
as a whole or a part of the delay circuits of the above-mentioned feedforward amplifier. As described above, in the manufacturing process of the feedforward amplifier, it is necessary that the delay time of the delay circuit can be fine adjusted. Therefore, the inventor of the invention studied what relationship exists between the electrical length and the delay time of the signals at the terminal
202
; that is, the electrical length of the transmission line
204
side viewed from the terminal-b of the circulator
203
(hereinafter, referred as “electrical length of the transmission line”) and the delay time of the signals at the terminal
202
. Furthermore, the inventor of the invention studied what relationship exists between the above-mentioned electrical length of the transmission line and the amplitude values of the signals at the output terminal
202
, and obtained the results as shown in FIG.
11
(A) and FIG.
11
(B).
That is to say, FIG.
11
(A) and FIG.
11
(B) indicate the fact that both the amplitude characteristic and the delay time characteristic changes largely with ripples relative to the changes in lengthy of the open-ended transmission line
204
connected with the terminal-b of the circulator (i.e., changes of the electrical length of the transmission line
204
side viewed from the terminal-b). The reason why such characteristics appear can be assumed that, in actual circulators, as described above, in many cases, because isolation between the terminals is not made sufficiently, the signals outputted at the output terminal-c appear as a composition of the signals from the path
1
and the signals from the path
2
shown in FIG.
10
.
Furthermore, the inventor of the invention discovered the problems for the first time that the leakage signals passing through the path
2
is approximately −15 dB to −20 dB, and that the leakage signals in manufacturing of the feedforward amplifier make it difficult to fine adjust the delay time. That is to say, as it is clear from FIG.
11
(A), changes of the delay time are large relative to the electrical length of the transmission line
204
. Furthermore, the electrical length of the transmission line
204
and the delay time are not in a proportional relationship. Therefore, it is understood that the delay time can not be fine adjusted, different from the case of the above-mentioned coaxial cable having a proportional relationship therebetween, by carrying out a simple work such as just cutting of the transmission line at a desired length or the like. In FIG.
11
(A) and FIG.
11
(B), the electrical length of the transmission line represented by the horizontal axis corresponds to a one-way distance of the transmission line
204
.
SUMMERY OF THE INVENTION
While taking into consideration the above described problems, the present invention was made to provide a delay circuit capable to reduce fluctuation in amplitude characteristic and delay time characteristic of the output signals, comparing to a conventio
Fujiwara Seiji
Ishida Kaoru
Matsuura Toru
Matsuyoshi Toshimitsu
Glenn Kimberly
Matsushita Electric - Industrial Co., Ltd.
Pascal Robert
RatnerPrestia
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